This invention relates to pumps for delivery of fluids to a patient. More specifically this invention relates to a fluid flow rate compensation system that monitors the tubing used in a pump in order to control the fluid flow rate of the pump or ascertain information about the fluid in the tubing.
Peristaltic pumps provide a pumping mechanism that pumps fluid through tubing by flexing or compressing the tubing repeatedly to achieve a desired flow rate. The flexing of the tubing induces wear that changes the geometric and mechanical properties of the tubing (e.g. elastic modulus, tan delta, wall thickness, diameters, etc.). These tubing properties directly impact the amount of fluid delivered in a pump stroke. Therefore by monitoring and understanding how the tubing is changing over time or over a plurality of pumping cycles, one can predict or compensate for the tubing wear which can result in improved pump performance.
One of the major factors in the peristaltic pumping flow rate is the geometry of the tubing. Because nothing can be manufactured perfectly there is a tolerance associated with the inner diameter (volume capacity) of the tubing as it comes from the manufacturer. A tolerance of +/−0.002 inches on 0.100 inches inner diameter tubing means a volume change of plus or minus 4%. For example, tubing that has an inner diameter of 0.102 inches will over deliver by 4% when it is initially used in the pump. If the pump knows the inner diameter of the tubing is 0.102 inches it can calculate the volume in the tubing, reduce the pump speed and achieve the desired flow rate.
A second geometric effect is caused by the tubing properties changing over time. It is known in polymer chemistry that as polymers degrade due to mechanical, environmental, or aging effects their polymer chains begin to break down, which results in a volume fraction decrease and increased densification. Even a piece of tubing sitting on the shelf will change over time and could affect the pump flow rate once installed in the pump. However, this effect is relatively small compared to mechanical degradation caused by the pump.
The most common technique for compensating for tubing wear is to include an algorithm in the pump that adjusts the mechanisms speed based on the amount of time the pump is running. Such an algorithm is developed based on running flow rate accuracy tests for extended periods of time. The pump is run on a single set mechanism speed and data is collected over time to show how the flow rate is affected as a function of time. This process is repeated over several set mechanism speeds such that a full characterization on how flow rate is affected by tubing wear. Once this data is obtained, an algorithm can be developed. Typically, for a given mechanism speed, flow rate begins to diminish over time as tubing wears. In order to compensate for this effect, the algorithm would adjust the mechanism speed (e.g. increase speed) such that steady flow rate could be maintained over time.
Several algorithms are known in the art. Nose et al., U.S. Pat. No. 7,284,956 describes in general one such invention using any number of feedback controllers, mechanism, sensors to maintain a pump flow rate at its operating set point. Another algorithm, specific to peristaltic pumping that is well known in the art is to utilize the internal pressure profiles which exist in the tubing.
Another technique for compensating for tubing wear is to implement sensors that characterize the fluid flow directly or indirectly and put it into a control system for closed loop feedback. Among the most common sensors that have been proposed for this are the indirect sensors. These sensors measure a characteristic that is indicative or can be correlated to the flow rate. Some of these common methods include: 1) measurement of tubing dimensions (inside diameter, outside diameter); 2) measurement of force to occlude/pump the tubing; or 3) measurement of the pressure inside/outside the tubing.
These techniques all rely on measuring properties that impact flow rate. Therefore monitoring how these properties change over time would also indicate how the flow rate is changing over time due to tubing wear. Another more rational approach is to utilize a sensor that directly indicates flow rate, a flow sensor. Many types of flow sensors exist (optical, ultrasonic, magnetic, etc). While these present one solution, many disadvantages exist for the medical device market because of the particularly high cost of implementation and clinical issues of these techniques.
Another way this problem has been overcome is to improve the tubing material itself. That is, to design and manufacture a tubing material that has reduced wear characteristics and can withstand the many cycles of compression that is undergone in peristaltic pumping. As such, materials such as silicone and Tygon™ are among the most popular for peristaltic tubing applications. As opposed to thermoplastic materials such as polyvinylchloride (PVC), they are highly resilient and compliant and have been shown to perform well in long term pump applications. Some disadvantages in this solution are the higher cost of this material, the difficulty in joining or attaching the tubing to other polymer components, and although the wear is greatly improved, the tubing still does degrade over time and is not a complete solution to the problem.
The most common current method of compensating for tubing wear in pump applications is to use an algorithm to offset the effect. In fact, many of the techniques presented are not currently known to be used in any medical pump device that is on market today.
Therefore, a principal object of the present invention is to provide an infusion system with a fluid flow rate monitoring and compensation system that improves the efficiency and accuracy of fluid flow through tubing over time.
Yet another object of the present invention is to provide a more cost effective fluid flow rate monitoring and compensation system.
Yet another object of the present invention is to utilize electrical properties of tubing to vary the flow rate of fluid through tubing.
Yet another object of the present invention is to utilize electrical properties of tubing and the fluid media flowing within the tubing to determine if there are gas or air bubbles in the fluid, breaks or leaks in the tubing.
These and other objects, features, or advantages of the present invention will become apparent from the specification and claims.